This invention relates to a oxygen sensor which is in principle an oxygen concentration cell having a solid electrolyte layer and useful for detecting oxygen concentration in gases and liquids, and more particularly to an oxygen sensor of this type having a construction suitable for use in either an intake system or an exhaust system of an automotive internal combustion engine.
Oxygen sensors of this type are of practical use for detecting oxygen content of, for example, molten metals and combustion engine exhaust gases. For this type of oxygen sensors, a reference oxygen partial pressure needs to be applied to one side of a solid electrolyte layer while the other side is exposed to a substance subject to measurement and the solid electrolyte layer must be kept at elevated temperatures to maintain the conductivity of the solid electrolyte at a high level. When the measurement of oxygen content of a gas whose temperature is not sufficiently high either constantly or temporarily, therefore, the oxygen sensor for the measurement needs to comprise a certain heating means. For this purpose, it is a usual practice to adopt a resistance heating method by the provision of a heater wire around the solid electrolyte layer so as to accomplish the heating by radiation and convection as shown, for example, in Japanese patent applications: Publication No. 47(1972)- 28957, Publication No. 49(1974)- 19839 and Public Disclosure No. 47(1972)-37599.
The resistance heating method in conventional oxygen sensors, however, is not satisfactory in efficiency, particularly when used in a gas stream of a large flow rate since a large amount of heat generated by the heater is carried away by the gas stream without being transferred to the electrolyte layer. The application of a greatly increased current to the heater as compensation for the heat loss is uneconomical and requires the provision of an unduly big power supply. Accordingly the conventional resistance heating method is unsuitable for practical use on vehicles typified by automobiles, so that the measurement of oxygen content of an air-fuel mixture in a fuel supply systems for automotive internal combustion engines as the basis for a precise control of the air/fuel ratio of the mixture has encountered a difficulty.
When an oxygen sensor comprising a solid electrolyte oxygen concentration cell is exposed at one side of its solid electrolyte layer to a substance whose oxygen partial pressure is P.sub.1 and at the other side to a reference substance providing a reference oxygen partial pressure P.sub.2, the cell develops an electromotive force (EMF) E across the electrolyte layer determined by the Nernst equation: EQU E = (RT/4F) log.sub.e (P.sub.1 /P.sub.2) (1)
where R is the gas constant, T represents an absolute temperature at which the solid electrolyte layer is kept, and F is the Faraday constant.
A practical output voltage V of the cell differs from the potential E since the cell has an internal resistance R.sub.1 and the potential E is detected by means of an instrument having an input resistance R.sub.2. The practical output voltage V is given by EQU V = [R.sub.2 /(R.sub.1 + R.sub.2)] E (2)
the solid electrolyte in the cell has such a great resistivity that the internal resistance R.sub.1 of the cell can be regarded nearly equal to the resistance r of the solid electrolyte layer, given by EQU r = (1/.rho.).multidot.(t/S) (3)
where .rho. is the conductivity of the solid electrolyte, t is the thickness of the solid electrolyte layer and S is an effective surface area of the same layer. The conductivity depends on the degree of ease in the migration of oxygen ions in the solid electrolyte and hence is a function of temperature: EQU .rho. = .rho..sub.o exp ( - [Q/RT]) (4)
where .rho..sub.o is a constant specific to each material, Q is another constant which is specific to each material and implies an activation energy for diffusion of ions, and both R and T represent the same as in Equation (1). Accordingly Equation (2) for the output voltage V is rewritten as ##EQU1##
Equation (5) indicates the output voltage V depends greatly on the temperature T.
Based on the recognition that a decrease in the thickness t of the solid electrolyte layer (a lowering in the internal resistance R.sub.1 of the cell) is effective for raising the output voltage V and for lessening the total heat capacity of the sensor (meaning more ease in raising the temperature T of the solid electrolyte), we have recently proposed an oxygen sensor of a novel construction. This oxygen sensor has a base plate of an electrically nonconductive material as a basic structural member of the sensor, a thin layer which is laid on one side of the base plate and is made of a mixture of a metal and an oxide of the metal, a thin solid elctrolyte layer formed on the metal-oxide layer and a thin and gas permeable cathode electrode layer laid on the electrolyte layer. The metal-oxide layer serves both as the source of a reference oxygen partial pressure and as the anode electrode layer though it is optional to interpose a thin metal layer as the anode electrode layer between the base plate and the metal-oxide layer. Since every component of the sensitive part of this oxygen sensor is made as a thin layer or film and the sensor includes the source of a reference oxygen partial pressure in the form of a thin layer of a solid material, the sensor can be produced as a very compact and physically strong device with a functional advantage of being quickly heatable.